1. Field of the Invention
The present invention relates to an image forming apparatus to be applied in, for example, light-emitting diode (LED) head and thermal head.
2. Description of the Prior Art
An optical printer head 201 of a typical prior art is shown in FIG. 1, and FIG. 2 is its longitudinal sectional view. A common lead wire 2 is formed on the surface of a substrate 1 made of electrically insulating material. Light-emitting diode (LED) elements 3 are joined in the upper part of the common lead wire 2, which works as an electrode for emitting light by applying an electric power to the LED elements 3. Parallel to the array of LED elements 3, driving circuits 4 are disposed on the substrate 1. Connection lead wires 5 are formed on the substrate 1. These connection lead wires 5 are connected to the individual electrodes 3b of LED elements 3 and output terminals 4a of the driving circuits 4 through bonding wires 6 individually, while input terminals 4b of the driving circuits 4 are connected to individual driving lead wires 7 on the substrate 1 by bonding wires 8. The individual driving lead wires 7 are connected to multiple printed wires 10 disposed on a flexible circuit wiring substrate 9. One LED array 3a comprises, for example, 64 LED elements 3, and a total of 40 such arrays 3a are disposed on the substrate 1. Therefore, the total number of LED elements 3 amounts to 2,560.
FIG. 3 is a block diagram showing a pracitcal electrical composition of a driving circuit 4. The driving circuit 4 is disposed on each array 3a. Shift registers 12 in a total of 64 bits possessing bits individually corresponding to LED elements 3 are connected in cascade, and clock signals shown on the top line in FIG. 4 are supplied to these shift registers 12 from lines 13. Synchronizing with these clock signals, print data is fed into the shift registers 12 from lines 14 as shown on the second line in FIG. 4. The store information of shift registers 12 is transferred and stored into latch circuits 16 corresponding to latch signals from lines 15 shown on the third line in FIG. 4. A strobe signal shown on the bottom line in FIG. 4 is given to a line 17, and accordingly the store information in the latch circuits 16 is led out into the output terminals 4a through an AND gate 18, so as to be individually applied to LED elements 3.
A total of 2,560 clock pulses are given to lines 13, and at the same time print data is given serially from lines 14, and in this way the print data for the portion of one scanning line is stored in a total of 40 shift registers 12, and the latch signals are given to lines 15, and the print data totaling to 2,560 are transferred in batch to the latch circuits 16, and 2,560 LED elements 3 for the portion of one line are selectively illuminated and driven on the basis of the print data for the duration period T of the strobe signal.
If emission outputs of plural LED arrays 3a are varied, and/or emission outputs of plural LED elements 3 contained in one LED array 3a are varied, plural times of emission driving are effected for each line in order to make uniform the emission outputs, that is, the exposure quantities by the LED arrays 3a or LED elements 3 having variations in emission outputs, for the exposure and printing of one line. In other words, the arrays 3a low in emission output and/or LED elements 3 low in emission output are selectively illuminated plural times depending on the print data.
For example, when composed to emit twice, in order to emit and drive in two emission drive periods, a total of 2,560 pieces of serial and sequential print data must be fed and stored in a total of 40 shift registers 12. If the emission outputs of arrays 3a or LED elements 3 have greater fluctuations, the emission driving must be repeated selectively more times, which results in lowering of the printing speed.
In this prior art, a total of 2,560 LED elements 3 for the portion of one scanning line are illuminated and driven for the duration of strobe signals selectively and in batch on the basis of the print data, and therefore a large current flows momentarily. Accordingly, a power supply with a large electric power capacity is needed.
Besides, a large Joule heat is generated in the LED elements 3, common lead wires 2, connection lead wires 5, and bonding wires 6, 8, and the temperature of LED elements 3 rises. As a result, the emission wavelength of LED elements 3 and the brightness vary depending on the temperature, and fluctuations occur in the emission wavelength and brightness in each LED element 3. Hence, clear printing is disabled, and uneven printing occurs.
To prevent temperature rise of LED elements 3, a heat sink may be used, but it causes the structure to be enlarged, and mounting is difficult in the recent electrophotographic printer and other recording apparatus in the tendency of reduction of size.
This prior art also involves other problems. For example, there are too many bonding wires 6, 8, and it takes a very long time for connecting them, and it may lead to a higher rate of defectives of connections.
As a further different problem of the prior art, the integrated circuits 4 are individually provided for each one LED arrays 3a, and therefore a great number of driving circuits 4 should be required.
Thus, in a coventional optical printer head 201, LED arrays 3a and driving circuits 4 are disposed correspondingly by 1:1, and the length w1 of the driving circuits 4 along the array direction is selected to be equal to the length w2 of the LED arrays 3a along the array direction.
FIG. 27 is a block diagram showing a structural example of an optical printer 51 using an optical printer head 201, and this diagram is also referred to in the embodiments. The optical printer 51 comprises a photosensitive drum 52 which is, for example, right cylindrical and is rotated and driven in the direction of arrow A1, and the photosensitive drum 52 is surrounded by a charger 53 for electrically charging the entire outer surface of the photosensitive drum 52, the optical printer head 201 for focusing an optical image on the photosensitive drum 52 to form an electrostatic latent image, and a developing device 54 for making the electrostatic latent image visible by using toner and others, and the toner image made visible is transferred on a recording paper 56 held against, for example, a transfer roller 55. The toner image on the recording paper 56 is fixed by a fixing device (not shown).
In the conventional optical printer head 201 described herein, the following problems are known.
(1) Since the lengths w2, w1 of the LED array 3a and driving circuit 4 must be defined to correspond to 1:1, LED arrays 3a of length W2 in plural types corresponding to the print dot density are prepared, and exclusive driving circuits 4 must be designed and manufactured accordingly. Therefore, the development requires an enormous labor and cost, and the production efficiency for fabrication of driving circuits 4 is lowered.
(2) When the LED arrays 3a are at relatively high density, it is necessary to bond by bonding wires 6, 8 at high precision for every LED array 3a as shown in FIG. 5, and the working efficiency deteriorates and the production efficiency are lowered.
(3) In the case of an optical printer head 201 composed as shown in FIG. 1, the length along the rotating direction A1 of the photosensitive drum 52 in FIG. 27, that is, the entire length of the optical printer head 201 including the length L1 in the vertical direction in FIG. 1 is extended. In other words, with respect to the central axis of the photosensitive drum 52 shown in FIG. 27, the angles .theta.1, .theta.2 formed by the optical printer head 201 with the charger 53 and developing device 54 become very wide.
FIG. 6 is a graph showing the time-course changes of the surface potential of the photosensitive drum 52, and FIG. 7 is a graph showing the relation between the surface potential of the photosensitive drum 12 and the toner deposition. At time t0 in FIG. 6, charging on the photosensitive drum 52 is started, and at time t1, reaching the specified quantity of charging, the charger 53 stops its operation. Afterwards, until exposure action at time t2, the surface potential attenuates spontaneously, and exposure is effected by the optical printer head 201 at time t2. In exposure, light is not emitted to the portion to be printed in black, and a spontaneous attenuation curve l1 continuous to curve l0 from time t1 to t2 is drawn. In the portion to be printed in white, the maximum quantity of light is emitted, and the surface potential drops suddenly as indicated by curve l2. In the portion of halftone gray printing, an intermediate quantity of light is emitted, and an intermediate curve l3 of curves l1, l2 is drawn.
Therefore, the angles .theta.1, .theta.2 will define the time intervals T.theta.1, T.theta.2 among times t1, t2, t3 in FIG. 6. Hence, the wider the angles .theta.1, .theta.2, the greater becomes the spontaneous attenuation quantity .delta.0 of the surface potential at exposure time t2, and the print quality deteriorates. Besides, between exposure time t2 and development time t3, if the attenuation .delta.1 is large, the print quality similarly deteriorates. To wit, as shown in FIG. 7, the toner deposition is limited by the surface potential, and printing at desired density is not realized if the degree of spontaneous attenuation is great, and coloring may be disturbed, specially in color printing.
(4) If the angles .theta.1, .theta.2 are relatively wide, even when the photosensitive drum 52 is reduced in size for downsizing the optical printer 51, the optical printer head 201 to be disposed in the periphery cannot be reduced in size, and hence it is difficult to reduce the size of the optical printer 51.
FIG. 8 is a sectional view showing an internal structure of an optical printer head 201 assumed in relation to the above problems, FIG. 9 is an exploded perspective view showing the appearance of the optical printer head 201, and FIG. 10 is a sectional view seen from line K1--K1 in FIG. 9. Referring to these drawings, the optical printer head 201 is explained below. In the optical printer head 201, plural light-emitting diode arrays (LED arrays) 203 are arranged in multiplicity on a straight line on a substrate 202 made of electrically insulating material, and on each one of the LED arrays 203, plural LEDs 204 are formed on a straight line parallel to the array direction of the LED arrays 203.
On the substrate 202, individual signal lines 205 for sequentially connecting the corresponding LEDs 204 of LED arrays 203 are formed, and an insulation layer 206 is disposed so as to partly cover the individual signal lines 5. On this insulation layer 206, common signal electrodes 207 are formed as many as the number of LED arrays 203, and the LED arrays 203 are formed on these common signal electrodes 207. Corresponding to each one of LEDs 204 of LED arrays 203, terminals 208 are provided, and the terminals 208 and individual signal lines 205 are connected together by bonding wires 209.
The common signal electrodes 207 are connected with electrodes 211 of flexible wiring substrate 212 integrally comprising flexible film 210 made of polyimide resin or the like and electrodes 211.
The substrate 202 on which the LED arrays 203 are mounted is disposed on a housing 213 having a function of cooling plate of optical printer head 201, and a lid 214 for pressing the flexible wiring substrate 212 is disposed between them. By this housing 213 and lid 214, the housing 215 of the optical printer head 201 is composed.
The lid 214 is provided with a pressing member 216 having an elasticity for adhering the flexible wiring substrate 212 to the substrate 202, especially to the common signal electrodes 207. To fix this lid 214 to the housing 213, penetration holes 217, 218 are formed in the lid 214 and flexible wiring substrate 212, and screw holes 213a are formed in the housing main body 213 to be engaged with the setscrews 219 passing through these penetration holes 217, 218. The flexible wiring substate 212 is connected to a connector 220, and is supported at a position remote from the housing 213 by a predetermined distance.
The first subject for the optical printer head 201 is as follows.
The insulation layer 206 is formed in order to prevent short-circuit between the individual signal line 205 and the common signal electrode 207, but because of the three-layer structure, in this conventional example, two steps of vapor deposition of the metal layer and two steps of etching are necessary for forming patterns of the individual singal line 205 and common signal electrode 207, and therefore the working time and necessary materials increase, and the product yield is lowered.
In particular, when forming common signal electrodes 207, a very high precision is needed for positioning with the previously formed individual signal lines 205, and an enormous time and high precision technique are required. Moreover, because of such positioning of high precision, it is difficult to meet the demand for increasing the size of the substrate 202, in the aspects of extension of length of optical printer head 201 and manufacturing efficiency of the substrate 202 for mounting LED arrays 203.
The second problem is as follows. Since the flexible wiring substrate 212 is press-fitted to the substrate 202 in the structure as described above, in order to mutually cover the flexible wiring substrate 212 and the substrate 20 to realize an electric conduction between electrodes, a length of about 2.0 mm is required for the covering range A1, and therefore the penetration holes 218 and screw holes 213a are required to have a diameter of about 3.0 mm. That is, downsizing of the optical printer head 201 is limited.
On the other hand, it may be also considered to omit the connection region A1 of the flexible wiring substrate 212, and connect the circuit wiring on the remaining flexible wiring substrate 212 and the substrate 202 by bonding wires, but in the optical printer head 201, the Common signal electrodes 207 possess, in the space opposite to the substrate 202, insulation layer 206 made of relatively soft material such as polyimide, and individual signal lines 205. When the bonding wires used in such common signal electrodes 207 are connected by ordinary technique, the common signal electrodes 207 which are relatively thin metal films on the insulation layer 206 made of relatively soft material are deformed in convex by the pressure of the jig used in connection at the bonding wire connecting points, and the bonding becomes difficult, and therefore this connecting technique is not employed.
FIG. 11 shows an electrical constitutional block diagram of the optical printer head 201. The parts correspondings to those in the foregoing prior art are identified with the same reference numbers. This optical printer head 201 of current changeover type driving system comprises a plurality of light-emitting diode arrays (LED arrays) g1, g2, . . . , gn composed of plural LEDs 204, and the LEDs 204 are arranged linearly. The corresponding LEDs 204 of LED arrays gi (i=1 to n) are individually connected to plural individual signal lines 205, and driving circuits 4 comprising transistors 221 for driving the LEDs 204 are connected to the individual signal lines 205. To the LED 204 selected by the driving circuit 4, a driving current from a power supply 222 installed separately is supplied through the individual signal lines 205, thereby realizing emission of a desired LED 204.
The cathodes of the LEDs 204 composing the LED arrays gi are commonly grounded, while the anodes are connected to the power supply 222 through the current limiting resistance R connected respectively in series. Between each LED 204 and the corresponding resistance R, the collector of the transistor 221 is connected, and the emitters of the transistors 221 are commonly grounded. Each LED 204 comprises wiring resistances R1, R2, . . . , Rn, and the wiring resistance is higher in the LED 204 as going remoter from the driving circuit 4. By feeding driving signal Sg to each base of the transistor 221, each LED 204 is individually lit and extinguished, thereby forming an electrostatic latent image as stated above.
When the LED 204 is not lit, the corresponding transistor 221 is set in conductive state, and the current from the power supply 222 is passed to the transistor 221 side, so that the current flowing into the LED 204 is cut off, thereby putting it out. On the other hand, when illuminating the LED 204, the transistor 221 is cut off, and the current from the power supply 222 flows into the LED 204.
In such optical printer head 201, since the individual signal lines 205 connected to the LEDs 204 are relatively thin and long, the wiring resistances R1 to Rn are large, and hence a voltage drop occurs due to the wiring resistances R1 to Rn. For example, when the LED array gn connected at a position remote from the driving circuit 4 which is the supply source of the driving current of the individual signal line 205 is selected, as compared with the case when the LED array g1 close to the driving circuit 4 is selected, the wiring resistance is added and increased, while the driving current becomes smaller. Therefore, the remoter is the LED array gi (i=1 to n) at the connecting position from the driving circuit 4, the less is the quantity of emission, and when such optical printer head 201 is used in an electrophotographic apparatus, the image quality deteriorates.
Incidentally, in the type of changing over the current between the LED 204 and the transistor 221 as mentioned above, the resistances R are used owing to the following reason. That is, assuming a case without this resistance R, when not illuminating the LED 204, if the power supply 222 is grounded through the transistor 221 in the conductive state, an excessive current flows into the transistor 221. The transistor 221 possesses an ON voltage VCE in conductive state between the collector and emitter, and when it exceeds the ON voltage of the LED 204, the LED 205 may be lit unexpectedly.
Moreover, in the above example, regardless of the emitting state or extinguished state of the LED 204, a current flows into the resistance R, and the power consumption in the optical printer head 201 increases, and the structure is incresed in size because it is necessary to install the resistance R in each LED 204.